Study On Accuracy Model And Assembly Process Optimization For Prismatic Pair Of Machine Tool

In this paper,the kinematic geometry is developed to the error motion of prismatic pair.The invariants and their characteristics of the error motion are defined and studied with the discrete measurement data.The applications of invariants in test,evaluation,accuracy analysis,process adjustment and calibration are discussed.(1)The evaluation based on invariants of error motion is proposed.As the motion of the rigid body is constrained by P-pair,the sufficient and necessary conditions are studied,while the kinematic property of the generatrix and moving axis are discussed.With the ideal motion and the error motion compared together,the global invariants are obtained,while the quasi moving axis and the quasi fixed axis are calculated.The invariant accuracy is then defined,which reveals the global kinematic property of prismatic pair and makes the evaluation more objective and comprehensive.(2)The elastic error constraint model for accuracy characteristics of prismatic pair is established.The geometric error of guide rails,the constrained structure of moving joints,the elastic deformation of part structures and the external load are considered synthetically.With the compatibility of deformation under elastic constraints researched,the function for the characteristic states of components and the actual spatial motion of moving table is revealed.This model can predict the invariant accuracy and its characteristics of prismatic pair precisely,which provide the basis for process adjustment and error calibration.(3)The assembly process optimization model for guide rails of prismatic pair is established.The statically indeterminate model of clamped beam is equivalent to the actual constraint of guide rail,then the elastic deformation function can be analyzed.In order to promote the invariant accuracy,the process optimization model with once/multiple adjustments for guide rails is established.The process parameters including distribution of screws,adjustment position and adjustment quantity are discussed,and the geometric modification scheme for the guide rails is presented.This process optimization model brings theoretical basis and convenience for machine tool assembly.(4)The error model for kinematic chain composed of prismatic pairs is established.Based on the elastic constrained structures and invariants of prismatic pairs,the structural parameters and the kinematic pair errors of the kinematic chain are defined respectively.The rule of error transmission and transformation under the action of geometric/physical parameters and external loads is studied.The function for the output error motion of the kinematic chain is obtained.This model is applied to a machine tool composed of three prismatic pairs,while the error characteristics in different cases are analyzed and the spatial trajectories of the function point are compensated.